The present invention relates to a programmable read only memory (hereinafter referred to as programmable ROM), and particularly to a programmable ROM employing semi-conductor non-volatile memory elements.
Conventionally known semiconductor non-volatile memory elements can be represented by floating gate avalanche injection MOS transistors (hereinafter referred to as memory MISFET's) in which a control gate is laminated on a floating gate via an insulating film.
A programmable ROM is formed by arraying, for example, a plurality of the above-mentioned memory MISFET's in the form of a matrix. Drains of the plurality of memory MISFET's arrayed along the same row are commonly connected to a bit line for writing and reading data, and control gates of the memory MISFET's arrayed along the same column are commonly connected to a word line.
A relatively high voltage is applied to selected bit lines to write the data, and a high voltage is also applied to the selected word lines. Accordingly, hot carriers are injected into floating gates of the memory MISFET's which correspond to the selected bit lines and word lines. In other words, the data is written in the selected memory MISFET's.
With reference to the non-selected memory MISFET's, in this case, parasitic capacity will arise between the drains and the floating gates. Therefore, when the potential of the drain is raised by the rise in the potential of the bit line, the potential of the floating gate is raised correspondingly. Consequently, the non-selected memory MISFET's are rendered slightly conductive despite the fact that the potential of the control gate is of the low level. Namely, a leakage current flows from the selected bit lines to the non-selected memory MISFET's. In other words, the writing current produced by the writing means is partly permitted to flow as the leakage current.
When the voltage applied to the bit lines is raised excessively, on the other hand, the non-selected memory MISFET's operate in the negative resistance region due to the high voltage, and this imposes the probability of breakage.